Heterogeneous photocatalysis is known since the 70's as a result of studies on the photo-induced dissociation of water. This technique consists in irradiating a semiconductor, usually of titanium dioxide, by natural sunlight or by artificial UV illumination, (λ<400 nm). This material undergoes excitation which allows an electron e− of the valence band to be ejected in the conduction band of the semiconductor (reduction). The corresponding hole h+ (oxidation) may react with an OH group adsorbed on the surface of the semiconductor in order to form highly oxidising hydroxyl radicals OH. These are capable of reacting with organic molecules, for example pollutants, leading to the mineralization of the latter with the formation of water and carbon dioxide. Photocatalysis is an acceleration of the photoreaction due to the presence of the catalyst. It is heterogeneous since the photoreactions are generated at the interface between two environments that are in different phases at the surface of the catalyst.
Titanium dioxide TiO2 exists in different crystalline forms: rutile, anatase, brookite and a large number of phases obtained by high pressure. Only the rutile and anatase crystalline forms show photocatalytic activity. In particular, anatase, the most active form, which was used in the frame of the present invention, has an elongated tetrahedral structure with irregular octahedrons of oxygen.
Titanium dioxide is present in great quantity, whether in paints, cosmetics, foodstuffs, etc. Its photoactivity thus allows to use it in order to decompose organic molecules adsorbed on its surface. Heterogeneous photocatalysis upon contact with titanium dioxide was therefore used in particular for the purification of water, destruction of pollutants, pesticides, dyes, bacteria, detoxification of agricultural and industrial rinsing waters, air purification (deodorisation, elimination of toxic gases) and as self-cleaning agent for objects or buildings in the open air, exposed to the elements.
A very large number of air purification and/or deodorisation devices with a photocatalytic filter based on TiO2, some of which also have a structural or architectural function, have been proposed. In most cases, the problems solved by these inventions are:                optimisation of the use of UV irradiation, as a result for example of the optimisation of the geometry and arrangement between the photocatalytic support and the lamps or by increasing the effectiveness of a UV lamp regarding its photocatalytic effect by the addition of reflectors (see for example JP 09 084866, EP 993859, JP 2000 334448, JP 10 249166, JP 2001 293336, JP 2002 295874, JP 2001 218820);        increase in the range of spectral light usable for catalytic activity, for example the use of visible light in applications of the photocatalytic light type (see for example JP 2002 083511, JP 2002 035599);        improvement in the maintenance of the devices resulting from the easier replacement of the lamps;        increase in compactness by the use of flat lamps or light-emitting diodes (LEDs) rather than traditional UV lamps (see for example JP 2000 051332, JP 09 000941).        
In most cases, the patented device is provided with a fan to force the circulation of air, the airflow possibly being made turbulent.
However, the devices according to the state of the art are of little or no assistance in meeting the following requirements:                incorporation of the air purifier as an architectural element requires for one thing, structural mechanical properties of the assembly that are satisfactory and for another, great compactness in terms of depth, which is difficult to achieve given the need to place the UV lamps with the correct orientation;        maximization of the surface of the photocatalytic filter achieved by UV illumination;        optimisation of the air circulation inside the device;        temperature regulation of the external walls of the device.        